1. Field of the Invention
The present invention relates to X-ray apparatuses, suitable for use in exposure systems, as well as in semiconductor device production.
2. Description of the Related Art
A method known as an X-ray demagnification (reduced magnification) projection exposure method has been suitably used in the production of devices having fine patterns, such as semiconductor circuit elements. According to this method, a mask carrying a circuit pattern formed thereon is illuminated with X-rays so that the mask image, i.e., the circuit pattern, is projected at a prescribed demagnification onto a wafer surface so that a resist on the wafer surface is exposed, whereby the circuit pattern is transferred to the resist in a reduced scale.
FIG. 10 shows an example of a conventional X-ray demagnification projecting exposure apparatus. This apparatus has, as its major components, an X-ray source 109, 110, 101, 111, an illuminating optical system 103, a reflective mask 104, a projection optical system 105, a stage 108 carrying a wafer 106, an alignment mechanism (not shown) for precisely aligning the positions of the mask 104 and the wafer 106, and a vacuum vessel and evacuating system (not shown) which cooperate with each other in maintaining a vacuum atmosphere around the entire optical system so as to prevent attenuation of t he X-rays.
Laser-excited plasma, for example, is used as the X-ray source. A laser source 109 emits laser beam in the form of pulses, which hit a target 111 so that laser plasma is generated. X-rays emitted from a luminescent point 101 of the laser plasma are collected on the reflective mask 104 through a collecting lens (not shown). The luminescent point 101 of the laser plasma has a size on the order of several hundreds of .mu.m and, hence, can be regarded as being a point source. The X-rays 102 emitted from the luminescent point 101 pass through filter 112 and are collimated by a parabolic mirror 103 having a focal point located on the luminescent point 101. The projection optical system 105 includes a plurality of multi-layered reflective mirrors which demagnify (reduce the magnification of) the pattern on the mask 104 so as to project the pattern image of a reduced size on the surface of the wafer 106. The projection optical system 105 is usually constituted by a telecentric system.
Conventional X-ray demagnifying projection exposure apparatuses, however, have suffered from the following problems.
Namely, these conventional apparatuses could not provide resolution and focal depth which would be sufficient for projecting extremely delicate and fine patterns on masks onto wafers, thus failing to transfer such patterns with a desired high degree of precision. In order to obviate this problem, it has been proposed to improve the image forming performance by using a phase shift effect offered by a phase shift mask. However, it has been impossible to fully enjoy the effects of such a phase shift mask. These problems encountered with conventional devices are attributable to the following reasons.
One of the characteristic parameters of an illuminating system is a coherence factor .sigma.. The coherence factor .sigma. is expressed as follows, representing the mask-side numerical aperture of the projection optical system by NA.sub.p1 and that of the illuminating optical system by NAi: EQU .sigma.=NAi/NA.sub.p1.
The optimum value of the coherence factor .sigma. is determined based on the levels of resolution and contrast required in the transfer of the pattern. In general, a too small value of the coherence factor .sigma. allows an interference pattern to appear at edges of the fine pattern image projected on the wafer, while a too large coherence factor .sigma. reduces the contrast of the projected image.
An illumination system having a coherence factor .sigma. of 0 (zero) is referred to as a "coherent illumination system". Such a coherent illumination system exhibits a constant optical system transfer factor (OTF) when the spatial frequency does not exceed a value given by NA.sub.p2 /.lambda. where NA.sub.p2 and .lambda. respectively indicate the wafer-side numerical aperture of the projection optical system and the wavelength of the X-rays. However, at higher spatial frequencies, the transfer factor OTF is zero, so that resolution of the image is impossible. In contrast, an illumination system having a coherence factor of 1 is referred to as an "incoherent illumination system". In this type of an illumination system, the transfer factor OTF decreases in accordance with an increase in the spatial frequency, but is not reduced to zero until the spatial frequency reaches a value which is given as 2.times.NA.sub.p2 /.lambda.. Thus, resolution is possible to a greater degree of fineness as compared with the coherent illuminating system. The conventional X-ray demagnifying projection exposure apparatuses, however, are designed such that the coherence factor .sigma. approximates 0 and, hence, operate under substantially coherent illuminating conditions. These apparatuses, therefore, have limited resolution and cannot transfer patterns having a high degree of fineness.